Commitments and Contingencies - Aircraft Purchase Commitments Narrative (Details) - Future aircraft purchase commitments - aircraft	1 Months Ended		3 Months Ended
	Dec. 31, 2018	Nov. 30, 2018	Dec. 31, 2018	Dec. 01, 2018	Oct. 31, 2018	Jun. 30, 2018
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	330		330
CRJ-900
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	15		15			20
A330-900neo
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	35	35	35		25
Number of aircraft with deferred delivery		10
A220
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required				90
Aircraft purchase commitments, increase	15
A220-100
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	36		36	40
Number of aircraft delivered			4
A220-300
Unrecorded Unconditional Purchase Obligation
Aircraft purchase commitments, minimum quantity required	50		50	50